1. Field of the Invention
The present invention relates to an oil-ring for disposition in an oil-ring groove of a piston of an internal combustion engine. More specifically, the invention relates to a composite oil-ring consisting of three ring pieces.
2. Description of the Prior Art
Various oil-rings of different types have been known for use with a piston of an internal combustion engine. Conventional oil-rings can be roughly classified by the following two types. One is a so-called solid oil-ring and the other is a three-piece type oil-ring. Generally, a solid oil-ring is constructed of, mainly, one ring piece which has an expansion force in its radial direction by itself and slidably contacts a cylinder bore. A three-piece type oil-ring is constructed of a pair of side rails and an expander disposed between the side rails and urging the side rails both radially toward a cylinder bore and axially toward the upper or lower surface of an oil-ring groove.
A solid oil-ring usually does not exert a force for sealing between the oil-ring and the upper or lower surface of an oil-ring groove, because one oil-ring is merely disposed in the oil-ring groove and thereafter inserted in a cylinder bore. Moreover, since the solid oil-ring itself has abutted end portions on its circumference, a gap between the abutted end portions forms an oil and gas path in a vertical direction, i.e. a direction along an axis of a cylinder bore.
Japanese Utility Model Publication SHO 48-13288 discloses an oil-ring structure designed to increase the axial sealing force of a solid type oil-ring, thereby reducing oil consumption in an internal combustion engine, as shown in FIG. 8. In FIG. 8, oil-ring 1 installed in oil-ring groove 2 of piston 3 consists of solid ring 4 and spring ring 5 having a belleville spring structure, which slidably contacts cylinder bore 6. Solid ring 4 is pressed onto the upper surface 2a of oil-ring groove 2 by the spring force of spring ring 5.
In the structure shown in FIG. 8, however, since spring ring 5 slidably engages cylinder bore 6 so as to follow the cylinder bore surface in the radial direction thereof, abutted end portions (not shown) of the spring ring must always have a gap in the circumferential direction. The gap between the abutted end portions forms an oil path in the axial direction, and it is difficult to prevent oil from escaping upwards through the oil path to a significant extent. Even if the location of the abutted end portions of spring ring 5 and the location of the abutted end portions of solid ring 4 are displaced from each other in the circumferential direction, it is difficult to decrease the oil loss through the oil-ring portion by a great extent because the oil which has passed through the abutted end portions of spring ring 5 and has entered into the interior of oil-ring groove 2 escapes upwards through the abutted end portions of solid ring 4. Particularly, it is recognized experimentally that oil escapes upwards through the oil-ring portion especially during engine braking, the oil having escaped burns during the next acceleration and evaporates, thereby increasing oil consumption.
In a three-piece type oil-ring, although there is a sealing force between a side rail and the upper or lower surface of an oil-ring groove which is generated by an urging force due to an expander, an oil path in the axial direction is inevitably formed at the abutted end portions of the side rail, because the side rail must follow a cylinder bore in the radial direction both by its own expansion force and by an expansion force exerted by the expander, and there must be a gap in the circumferential direction between the abutted end portions of the side rail in order to ensure that the side rail follows the surface of the cylinder bore.
Therefore, also in the three-piece type oil-ring, there is a problem similar to the problem in a solid oil-ring, with respect to oil loss through the oil-ring portion.